This invention relates to the field of synchronization in a communication system, and, more particularly to a method and system for synchronization between a transmitter and a receiver in a GSM communication system.
Global System for Mobile communications (GSM) is a widely used telecommunications standard. According to it, a transmitter transmits a modulated signal at particular points in time. To communicate with the transmitter, a receiver interprets a received signal for establishing timing and frequency synchronization with the transmitter. For example, when a user switches on a mobile cellular phone to communicate with a base station, the mobile phone receiver has to synchronize the timing and the frequency with the base station transmitter. The GSM system defines two channels for this purpose: a frequency control channel (FCCH) that helps in identifying the frequency of the received signal, and a synchronization channel (SCH) that helps in identifying the system timing. The FCCH broadcasts a frequency burst (FB), while the SCH broadcasts a synchronization burst (SB). The FB is detected at the receiver for frequency synchronization with the transmitter, whereas the SB is detected at the receiver for time synchronization with the transmitter.
The FB is a pattern of 148 zeros, with a sampling rate of 271 kHz, modulated according to the Gaussian minimum shift keying (GMSK) scheme. The FB is transmitted by using a radio frequency (RF) carrier. This results in a pure sinusoid, whose frequency is equal to the carrier frequency offset by one-fourth the sampling rate, i.e., 67.7 kHz. In other words, after the base band conversion, the FCCH appears as a complex sinusoid of frequency 67.7 kHz at the receiver.
However, the frequency of the received FCCH can be offset from 67.7 kHz because of one or more of the following effects. There may be frequency offsets between the transmitter and the receiver, due to imperfections or variations in either of them. There can be a Doppler effect in the environment, due to the movement of the transmitter or the receiver or other disturbances resulting in a shift in the frequency of the received RF carrier. This also causes a frequency offset in the base band signal. The value of this offset is unknown, making the detection of the FCCH or FB a complex problem.
In one of the methods relating to the detection of the FB in a received signal, the received signal is derotated by 67.7 kHz and then low-pass filtering is carried out. Derotation is defined as a process for correction of the frequency by the offset. The ratio of the output power to the input power of the filter is then computed. If it is above a certain threshold, an FB is detected in the received signal.
Some methods for the detection of the FB make use of the cross correlation between the received signal and a reference pure sinusoid at 67.7 kHz. If the value of the correlation is above a preset threshold, the FB is detected in the received signal. In one of the methods, a block of 148 samples is selected and divided into a number of smaller blocks, such that the maximum difference in the phase offset between the received signal and the reference signal over each block is less than 180°. The received signal and the reference signal are correlated separately in each of these blocks, and the values of correlation are added non-coherently. However, this non-coherent addition results in degradation in performance. Alternatively, the received signal can be correlated with a number of reference signals, with frequencies in the expected range. This method, however, becomes computationally complex for high frequency offsets. Another group of FB detection methods makes use of variants of autocorrelation, such as complex autocorrelation and correlation between the real and imaginary components of the received signal. These methods are, however, less accurate in the absence of large frequency offsets.
The conventional methods described above suffer from one or more of the following limitations. Certain methods are sensitive to frequency offsets. These methods only describe the detection of the FB for synchronization purposes. However, the synchronization process involves detection of both the FB as well as the SB. The conventional methods perform detection of the SB at a later stage. This makes synchronization a two-stage process. In the first stage, the FB is detected at the receiver for frequency synchronization with the transmitter. In the second stage, the SB is detected at the receiver for time synchronization with the transmitter. The first stage of the method ensures that the initial frequency offset in the received signal is corrected before decoding the SB. However, the process of synchronization takes a longer time, using this two-stage method. In addition, there may be cases of false detection of the FB. If this occurs, the time to achieve the synchronization increases further.
Accordingly, there is a need for a method and system that reduces the time required for synchronization. The method should also ensure that there are minimal cases of false detection of the FB. In addition, the method should provide synchronization, even under large frequency offsets.